420 



GENERAL ZOOLOGY 



full meal of blood may suffice for a year. Eyes and other sense 

 organs are present upon the head. Leeches are monoecious 

 and development is direct (Fig. 243). 



Examples: Hirudo medicinalis, the medical leech, a European 

 form used in blood letting, is now found in ponds and streams 

 in this country. Macrodella decora is one of the larger fresh- 

 water species. 



PHYLUM 14— ARTHROPODA 



Arthropods, like annelids, are segmented externally, and the 

 arrangement 'of the nervous system, muscles, heart, and other 

 organs shows evidence of internal segmentation. On the other 



1 



Fig. 244. — Appendages of crayfish of left side, ventral view, three-fourths 

 natural size. 1, first antenna; 2, mandible; 3 and 4, first and second maxillae; 

 5, 6 and 7, maxillipeds; 8, pleopod; 9, first walking leg (biramous). 



hand, arthropods differ from annelids in that (1) the segmentation 

 is heteronomous, which means that the primitive segments are 

 fused to form larger segments, such as the head, thorax, and abdo- 

 men; (2) the paired appendages are jointed; and (3) the body 

 cavity is almost eliminated. The possession of jointed appendages 

 is the most important feature and gives the name to the phylum ; 

 Arthropoda means jointed feet. These appendages were pri- 

 marily locomotor and sensory in function, functions which some of 

 them have retained, while others have evolved into (1) mandibles 

 and maxillae, chewing organs; (2) maxillipeds, intermediate 

 between legs and jaws; (3) pleopods, the swimmerets of Crustacea; 

 (4) spinnerets, the spinning organs of spiders; and (5) the ovipositor 

 of insects, etc. (Fig. 244). 



The chitinous exoskeleton is a secretion of the epidermis and 

 serves as a protective covering to which muscles and other 



